State whether the following is true or false.
All the cells of a multicellular organism contain the same genome.
Answer: C
Differentiation works by expressing different subsets of the same genes rather than by discarding genes outright. The mature red blood cell and the antibody-producing lymphocyte are the classic exceptions, having lost or deliberately rearranged their DNA during maturation.
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A bacterial culture doubles every 20 minutes. Starting from 1,000 cells, how many are present after 2 hours?
Answer: D
Two hours contains six doubling periods, and 2 raised to the sixth power is 64, giving 64,000 cells. Multiplying the starting figure by six, rather than doubling it six times over, is the usual mistake and gives a badly low answer.
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How long would that culture take to grow from 1,000 to about 256,000 cells?
Answer: A
Correct answer: 2 hours 40 minutes
The culture doubles every 20 minutes, and \[ \frac{256{,}000}{1{,}000} = 256 = 2^8, \] so eight doublings are needed: \( 8 \times 20 = 160 \) minutes, or 2 hours 40 minutes. Recognising 256 as a power of two turns the whole calculation into a count of doublings and removes any need for logarithms.
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The resolution limit of a light microscope is set principally by
Answer: B
Correct answer: the wavelength of the light used
Resolution is bounded by diffraction: \[ d = \frac{0.61\lambda}{\mathrm{NA}}, \] so with visible light around 500 nm and the best objectives, about 200 nm is the limit. Brighter illumination and stronger eyepieces enlarge the image without separating anything further, which is exactly the distinction between magnification and resolution.
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State whether the following is true or false.
Increasing the magnification of a light microscope indefinitely reveals ever finer detail.
Answer: C
Correct answer: False
Past the diffraction limit further magnification is empty magnification: the image grows but no new detail appears, because two points closer than about 200 nm cannot be separated by visible light however much they are enlarged. What is needed is a shorter wavelength, which is what the electron microscope supplies.
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An electron microscope achieves higher resolution than a light microscope because electrons
Answer: D
Correct answer: have a much shorter wavelength than light
An electron accelerated through 100 kV has a wavelength of a few picometres, some hundred thousand times shorter than visible light, and since the diffraction limit scales with wavelength the attainable resolution improves accordingly. In practice lens aberrations keep electron microscopes well short of that theoretical figure, but they still resolve to atomic dimensions.
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A transmission electron microscope differs from a scanning electron microscope in that the transmission instrument
Answer: A
Passing the beam through reveals internal structure in a very thin slice, while the scanning instrument reflects electrons off a coated surface to give its characteristic three-dimensional appearance. Neither instrument can examine living material.
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Consider the following statements about cell size.
1. A typical bacterium is smaller than a typical animal cell.
2. The surface area to volume ratio falls as a cell grows.
3. Cell size is limited chiefly by the strength of the plasma membrane.
Answer: B
The first two are correct. The third names the wrong constraint: the limit is set by the need to exchange materials fast enough across a surface that grows more slowly than the volume it must serve, so a cell reaches a ceiling long before its membrane fails mechanically.
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Which of these is the largest?
Answer: C
A eukaryotic cell of ten to a hundred micrometres dwarfs a bacterium of one or two, which in turn dwarfs a virus of a few tens of nanometres and a ribosome smaller still. The scale spans about four orders of magnitude.
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State whether the following is true or false.
Cyanobacteria are eukaryotic because they carry out photosynthesis.
Answer: D
They are bacteria, with no nucleus and no chloroplasts, and they photosynthesise on internal thylakoid membranes instead. The endosymbiotic theory in fact holds that chloroplasts descend from ancestors of this very group of organisms, taken up long ago by an ancestral eukaryote.
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